LAGOS, Nigeria – An anti-piracy watchdog says five sailors have been kidnapped off the coast of Nigeria's oil-rich southern delta.

The International Maritime Bureau said in a note Monday that the five sailors had been kidnapped Thursday some 45 nautical miles off the coast of Brass.

The bureau said the pirates came on a speed boat and a supply vessel, attacking the sailor's container ship. The bureau said the pirates kidnapped the five sailors and left the rest of the crew unharmed.

The Gulf of Guinea, which follows the continent's southward curve from Liberia to Gabon, has seen an escalation in violent pirate attacks. Insurers have listed Nigeria, neighboring Benin and nearby waters in the same risk category as Somalia.